Solar power systems offer much promise for clean energy, with few, or zero, carbon emissions. These systems collect incident sunlight and convert this sunlight into a usable form of power, such as electricity. Solar energy offers a clean, inexhaustible, sustainable solution to energy demands and has the potential to supply a very significant fraction of U.S. and global electricity consumption. While the U.S. and global solar power potential is known to be immense, solar power systems have not been economically competitive without government support, to date. Challenges remain to devise solar technologies that can lower installation costs, increase power output, and lower the marginal cost per unit energy produced, for a lower levelized cost of energy. An important metric is the overall system efficiency, that is, the electric power output per incident solar power collected.
Solar power systems include photovoltaic systems, solar thermal systems, and others. Photovoltaic systems utilize photovoltaic solar cells that convert sunlight directly into electricity by the photovoltaic effect. These solar cells are expensive, and their efficiencies are limited because they can exploit only a portion of the solar spectrum. These systems are also characterized by a large energy-payback period, i.e., the time they must be exposed to sunlight and produce electricity, to return the energy required to produce and install them.
Solar thermal systems convert sunlight into heat and use this heat to generate electricity. Examples of solar thermal systems include solar power towers, parabolic trough systems, and dish-Stirling systems. Solar power towers utilize a large number of steerable, planar, or near-planar mirrors that reflect and direct rays of sunlight to a central tower where a heat-transfer fluid is heated. The heat collected is typically transferred to rotating machinery, such as a steam turbine, that is used to drive an electric generator. These systems suffer from low efficiencies because of high optical losses, such as cosine and other optical losses, solar-receiver losses, as well as temperature and power losses from long fluid-flow loops to and from the tower. Cosine losses refer to the energy lost when light rays from the sun do not strike the mirror perpendicular to its surface. To reflect rays of sunlight to the central tower, individual mirrors form an acute angle to the sun, therefore requiring more mirror surface than when the mirror is perpendicular to the sun's rays. Collection efficiency is increased and mirror cost is less when the mirror is perpendicular to the sun.
Parabolic-trough systems utilize elongated cylindrical mirrors to heat a thermal fluid that is pumped through a pipe positioned on the focal line of each mirror. These systems are characterized by low thermal efficiencies because the operating temperature of the circulating thermal fluid is limited to avoid its decomposition. Additionally, these systems pay efficiency penalties because of the pumping power required to circulate the hot fluid around the field, and they suffer heat losses because the hot fluid circulates over long distances to cover the collector field before it can be used to heat the working fluid (typically steam) of the turbine-driven electric generator that produces electric power. Heat is lost through dissipation along these long flow distances.
Dish-Stirling systems utilize axisymmetric parabolic solar collectors, where each individual collector has its own power converter unit that generates electric power (e.g., a Stirling engine) supported at the focal point of the dish collector. While this system offers higher collection and conversion efficiency, it requires a very large number of individual engines to drive electric generators. Secondly, a complex and heavy structural mounting system is required to suspend the heavy generator at each collector's focus. Additionally, dish-Stirling systems are typically designed to produce power directly because it is difficult to adapt them to exploit thermal-energy storage to tailor their power-production profile to better match the desired power-demand profile, average output during cloud cover, and for other reasons, such as optimizing revenue according to terms specified in the electricity-generating plant power purchasing agreement with the electric grid utility.
A further issue in dish-Stirling solar-thermal systems is the inability to output power at levels required to operate turbines and other high-efficiency and high-reliability electric generators at optimum levels. As a result, dish-Stirling systems use reciprocating piston machinery characterized by high operational and maintenance costs.
Accordingly, there remains a need for concentrating solar power systems with thermal-energy storage options, capable of grid-scale electric-power output, high system efficiency, and low levelized cost of energy.